The present invention is related to the field of magnetic resonance.
More specifically, the invention is related to a spin resonance spectrometer comprising a radio frequency transmitter operating at a predetermined basic frequency, and a probehead supplied by the radio frequency transmitter.
Spin resonance spectrometers are well-known in the art in various configurations. In the subsequent description the invention shall be illustrated by way of an example, namely a nuclear resonance spectrometer, however, it goes without saying that other spin resonance or magnetic resonance phenomena are also encompassed by the invention, for example electron spin resonance.
Further, the term xe2x80x9cspin resonance spectrometerxe2x80x9d shall be understood to also comprise apparatuses in which magnetic resonance is utilized in the course of imaging processes, for example in nuclear spin tomography.
In nuclear resonance spectrometers the radio frequency signal is normally fed to the probehead in a gated or pulsed manner. For that purpose a signal of a predetermined basic frequency, normally in the range of several 100 MHz is generated by means of a synthesizer and is then gated by means of a programmable pulse generator according to a predetermined program or pulse sequence, and is subsequently amplified. The amplified signal is routed to the probehead via a low pass filter or via a band pass filter tuned to the basic frequency. The probehead is located within a magnet system and comprises the sample under investigation.
If the probehead is mistuned, harmonics of the basic frequency are generated. These harmonics are reflected from the probehead to the radio frequency transmitter. They enter the radio frequency transmitter at the final power stage and may cause damages or at least a shut down of the transmitter.
It is, therefore, an object underlying the invention to avoid these drawbacks. Accordingly, a spin resonance spectrometer shall be made available that is capable of operating even when the probehead is mistuned relative to the radio frequency transmitter without resulting in an emergency shut down of the radio frequency transmitter or even in damages.
According to the invention this object is achieved in that at least one load is interposed between the radio frequency transmitter and the probehead, the load being tuned to a multiple of the basic frequency.
The object underlying the invention is thus entirely solved.
The harmonics reflected to the radio frequency transmitter in conventional spectrometers, in particular the most hazardous third harmonic is fed to a load so that it may not result in damages within the radio frequency transmitter.
In a preferred embodiment of the invention this is achieved in that at least one load is connected to a junction between the radio frequency transmitter and the probehead via a first filter being tuned to pass a multiple of the basic frequency. The filter may be a band pass filter tuned to the multiple of the basic frequency or may be a high pass filter, the transition frequency of which being below the multiple of the basic frequency.
These measures have the consequence that a harmonic reflected from the probehead or from a low pass filter between the radio frequency transmitter and the probehead is directly routed after its reflection to the afore-mentioned junction or network node and is, hence, inactivated.
Preferably, the load is a 50xcexa9 load.
According to further embodiments of the invention, the radio frequency transmitter has an output provided with a second filter tuned to pass the basic frequency. The second filter may either be a band pass filter tuned to the basic frequency or may be a low pass filter, the transition frequency of which being above the basic frequency.
These measures, known per se have the advantage that the probehead is protected against harmonics.
According to a first alternative of this preferred embodiment of the invention, a junction is provided directly at the output of the radio frequency transmitter. However, as another alternative or additionally, a junction may be provided at the output of the second filter.
These measures have the advantage that according to the particular situation a first filter may be connected directly to the output of the radio frequency transmitter or to the output of the second filter.
The simultaneous provision of a first and a second filter has the advantage that a diplexer or frequency divider network is configured in which the basic frequency is directly routed from the radio frequency transmitter to the probehead and the measuring signals generated within the probehead may directly return via a transmitter/receiver switch to a receiver channel. The diplexer thereby guarantees that the hazardous harmonics reflected from the first filter are deviated from the transmitter output.
If, according to the above-mentioned second alternative, a first filter is connected to a junction at the output of the second filter, a harmonic generated within the probehead and reflected therefrom is deviated from this junction. This is particularly useful for such probeheads incorporating non-linear components as, for example, superconducting radio frequency coils.
It is, further, particularly preferred when the load is configured broad band, irrespective of its particular location.
This measure has the advantage that reflected spurious signals may be absorbed independent of their specific frequency.
In other preferred embodiments of the invention the load and/or the first filter or the first filter and/or the second filter, respectively, are structurally integrated into the probehead.
In a particularly preferred modification of this embodiment the second filter together with the load are integrated at the input of the probehead, preferably as a resonance circuit having an integrated resistive load. By doing so, it is avoided that the harmonics generated within the probehead are reflected between the probehead and a first filter located outside the probehead.
The spectrometer according to the invention may be equipped with a resistive magnet system or with a superconducting magnet system.
The spectrometer measuring frequency is preferably several 100 MHz.
In embodiments of the invention the probehead comprises a transmitter-and-receiver resonance system. These resonance systems usually comprise radio frequency resonance circuits having radio frequency coils and resistors or resonators. The resonance systems, irrespective of the particular design of the magnet system may as a whole or partially consist of a super-conducting material and may be operated at corresponding super-conducting temperatures.
These measures have the advantage that a high signal-to-noise ratio may be achieved.
The invention is preferably used within single channel spin resonance spectrometers. However, the invention may also be used in multi channel spin resonance spectrometers which, as known per se, comprise a multitude of radio frequency transmitters operating at predetermined basic frequencies and being allocated to a corresponding multitude of transmitter and/or receiver units within the probehead. In that case it is particularly preferred when at least one each load tuned to a multiple of the basic frequency is interposed between at least two of the radio frequency transmitters and the probehead. Moreover, the spectrometer is preferably configured according to one of the afore-described other embodiments of the invention.
Further advantages may be taken from the description and the enclosed drawing.
It goes without saying that the features mentioned before and those that will be explained hereinafter may not only be used in the particularly given combination but also in other combinations or alone without leaving the scope of the present invention.